The present invention relates to a structural body and to a hollow extruded frame member on which friction stir welding is carried out. For example, the invention relates to a railway vehicle, a building structure and others structures which are formed at least in part using aluminum alloy hollow extruded frame members that are joined using friction stir welding.
Friction stir welding is a method in which, by rotating a round rod (called xe2x80x9ca rotary toolxe2x80x9d), inserting the rotating rod into a welding portion, and moving the rotary tool along a welding line of a hollow extruded frame member, the welding portion is heated, softened and plastically fluidized and is solid-stately welded.
The rotary tool used in friction stir welding is comprised of a small diameter portion, which is inserted into the welding portion during welding, and a large diameter portion, which is positioned mainly outside of the welding portion during welding and is integrated with the small diameter portion of the rotary tool. The small diameter portion and the large diameter portion of the rotary tool have the same axis. A boundary between the small diameter portion and the large diameter portion of the rotary tool is inserted a little into the welding portion during welding.
The above-stated technique is disclosed, for example, in Japanese application patent laid-open publication No. Hei 9-309164 (EP 0797043 A2).
One difference between friction stir welding and arc welding is that, in friction stir welding, insertion of the rotary tool into the welding portion causes a large load to be applied to the hollow extruded frame members during welding. This load acts mainly in the insertion direction (an axial center direction) of the rotary tool. In other words, this insertion force of the rotary tool acts directly on the hollow extruded frame members to be subjected to welding.
When a hollow extruded frame member, having two face plates and ribs for connecting the two face plates, is subjected to friction stir welding, the above-stated insertion force acts on the face plate in an area surrounding the welding portion and the rib for connecting the face plates, causing the joint portion to deform. As a result, it is necessary to take steps to prevent such a deformation of the joint portion.
A deformation prevention means is disclosed in the above-referenced Japanese application patent laid-open publication No. Hei 9-309164 (EP 0797043 A2). This deformation prevention means is based on the provision, in the area of the welding portion, of a vertical plate (called a longitudinal rib) for connecting the two face plates. This longitudinal rib, which serves as one of ribs for connecting the two face plates, is arranged with respect to the welding line so as to be disposed on an axial center of the rotary tool. With this method, since the longitudinal rib supports the above-stated insertion force, the deformation of the joint portion can be prevented.
However, since a large load acts concentrically on the longitudinal rib, a high stress is generated at the connection portion of the longitudinal rib with the face plate, at which point the generated stress concentration exists. As a result, there is fear that the force will be sufficient to destroy the longitudinal rib.
As one means for preventing damage to the longitudinal rib, there is a method in which a visible outline of the connection portion between the longitudinal rib and the face plate is formed with a circular arc having a large diameter, which connection portion is recessed inside of the hollow extruded frame member, causing the longitudinal rib and the face plate to be connected smoothly, so that the stress concentration on the connection portion between the longitudinal rib and the face plate is spaced out.
However, the larger the diameter of the above-stated circular arc is, the more the cross-sectional area of the joint portion increases, and, accordingly, the weight of the joint portion increases. For these reasons, in the above-stated method, there is a limitation on the light weight characteristic of the hollow extruded frame member. In other words, there is a limitation on the light weight characteristic of the structure of the structural body which is comprised of these hollow extruded frame members.
An object of the present invention is to provide a structural body and a hollow extruded frame member, in which a light weight structure in the form of a member having two face plates and ribs for connecting the two face plates can be attained.
The above-stated object can be attained by a structural body comprising a first member having two face plates in the form of sheets, the one of the two face plates and the other of the two face plates being connected by a rib, and a second member which is subjected to friction stir welding to a connection portion of the other of the two face plates of the first member and the rib. At least one visible outline of a connection portion of the rib and a respective one of the two face plates is comprised of a curved line which is recessed in a solid side of the first member, the curved line being constituted by a combination of two continuous circular arcs, and within the two circular arcs, the diameter of the circular arc joined to a side of the said face plate is smaller than the diameter of the circular arc joined to a side of the rib.